Non-Dispersive infrared (NDIR) gas analyzers have been used for detecting the presence and concentration of various gases for over four decades. The NDIR technique has long been considered as one of the best methods for gas measurement. In addition to being highly specific, NDIR gas analyzers are also very sensitive, stable and easy to operate and maintain.
In contrast to NDIR gas sensors, the majority of other types of gas sensors today are in principle interactive. Interactive gas sensors are less reliable, generally nonspecific, and in some cases can be poisoned or saturated into a nonfunctional or irrecoverable state.
Despite the fact that interactive gas sensors are mostly unreliable and that the NDIR gas measurement technique is one the of best there is, NDIR gas analyzers have still not enjoyed widespread usage to date mainly because of the fact that their cost is still not low enough as compared to other inferior gas sensors for many applications.
In the past, NDIR gas analyzers typically included an infrared source, a motor-driven mechanical chopper to modulate the source, a pump to push or pull gas through a sample chamber, a narrow bandpass interference filter, a sensitive infrared detector plus expensive infrared optics and windows to focus the infrared energy from the source to the detector. In an attempt to reduce the cost and simplify the implementation of the NDIR methodology, a low-cost NDIR gas sensor technique was earlier developed. This low-cost NDIR technique employs a diffusion-type gas sample chamber of the type disclosed in U.S. Pat. No. 5,163,332, issued on Nov. 17, 1992 to Wong, one of the present applicants. This diffusion-type gas sample chamber eliminates the need for expensive optics, mechanical choppers and a pump for pushing or pulling the gas into the sample chamber. As a result, a number of applications using NDIR gas sampling technique, which were previously considered impractical because of cost and complexity, have been rendered viable ever since.
In the ensuing years since the U.S. Pat. No. 5,163,332 (1992) was issued, Wong, one of the present applicants, continued to refine and improve low-cost NDIR gas sampling techniques as evidenced by the issuance of U.S. Pat. No. 5,222,389 (June 1993), U.S. Pat. No. 5,341,214 (August 1994), U.S. Pat. No. 5,347,474 (September 1994), U.S Pat. No. 5,453,621 (September 1995), U.S. Pat. No. 5,502,308 (March 1996), U.S. Pat. No. 5,747,808 (May 1998), U.S. Pat. No. 5,834,777 (November 1998) and U.S. Pat. No. 6,237,575 (May 2001) to same. However, it has been quite apparent that despite the intense efforts over the years by Wong and others in the field, the unit sale price of NDIR gas sensors is still too high for many applications. It is of interest to note that back in 1991 and prior to the issuance of U.S. Pat. No. 5,163,332 (1992) to Wong, the same inventor has earlier advanced the concept of a simpler NDIR sensor methodology using spectral ratioing technique with a differential temperature infrared source in U.S. Pat. No. 5,026,992 (1991). However, even after almost 15 years, this concept has to date neither been proven to be viable in theory nor has it been experimentally demonstrated to illustrate its practicality. It was found out only very recently and experimentally by Wong, the original inventor of U.S. Pat. No. 5,026,992 (1991) and one of the present applicants, that although the concept as suggested by the author was sound, the method does not work if the prescribed steps were followed exactly according to the teaching of the patent. Furthermore, it was found out by the present applicants that the methodology itself has to be completely reformulated taking into consideration the shortcomings of both the method and the system components as suggested by the original inventor.
There is still a long felt need in a variety of industries and applications to use lower cost NDIR gas sensors, and so far this desire has gone unanswered. It is this need that the current application seeks to address and bring about a new and novel technique for the design and implementation for ultra low cost NDIR gas sensors.